Gray iron is an alloy of iron, carbon and silicon, which typically contains small amounts of other elements and residual impurities, such as manganese, phosphorous and sulfur. The carbon in gray iron is predominantly present as free carbon in the form of Type A flake graphite, which is characterized by long, thin flakes of graphite that are randomly oriented and dispersed throughout the iron matrix. In general, the graphite content of gray iron is a function of the carbon equivalent (C.E.), which may be calculated in percentage by weight as:C.E.=(% Carbon)+⅓(% Silicon+% Phosphorous)
The presence of Type A flake graphite provides gray iron with excellent thermal conductivity. In addition, gray iron is relatively inexpensive to produce and has good castability and machineability. Gray iron also has good vibrational damping capacity. Thus, gray iron has a number of mechanical and physical properties which make it suitable for use in a variety of applications. For example, gray iron is particularly useful in the manufacture of automotive components such as brake drums and brake rotors, which require materials having good thermal conductivity to resist thermal fatigue caused by heating during braking.
One drawback to gray iron is that it has relatively low tensile strength and, therefore, articles made of gray iron typically have larger and heavier structures than would be necessary if the same component could be made of other materials. The need for lighter weight gray iron products is particularly acute in the trucking industry, which is subject to state and municipal codes limiting the weight of commercial vehicles. For example a 20% reduction in the weight of a gray iron brake drum can result in an overall weight saving of as much as three hundred pounds for a tractor-trailer having multiple axles.
Various grades of gray iron exist that have increased tensile strength, which would permit lightweight product designs. However, grades of gray iron having higher tensile properties also have a lower carbon equivalent and, therefore, reduced graphite content. The carbon equivalent of such high tensile strength gray irons is typically below the range of about 4.10% to about 4.25% which must be maintained for good thermal conductivity and thermal shock resistance. For example, pursuant to ASTM specifications (American Society of Testing Materials), gray irons of Class 40, 45, 50, 55 and 60 having tensile strengths of 40,000 psi and greater all have a maximum carbon equivalent of 3.80%. In addition, the decrease in graphite content also reduces the castability, machineability, and vibrational damping capacity of gray iron. Thus, increasing the tensile strength of gray iron sacrifices its desirable mechanical and physical properties.
Various attempts have been made to increase the tensile strength of gray iron by the addition of other elements, to permit the design of lightweight cast iron products such as brake drums. Ductile iron is formed by the addition of approximately 0.02% to 0.04% magnesium (by weight) to cast iron, which causes the graphite to assume a spheroidal or nodular form. The change from flake to nodular graphite increases the tensile strength of ductile iron to >65,000 psi in comparison to gray iron that is typically used for brake drum applications, which has a tensile strength of only about 30,000 psi. However, the presence of nodular graphite significantly decreases thermal conductivity of ductile iron in comparison to gray iron containing Type A flake graphite.
In the case of brake drums, reduced thermal conductivity also reduces the ability of the brake drum to absorb and then dissipate the heat generated during braking. Repeated and/or prolonged braking creates thermal stress, which can cause the braking surface to deform and form ridges that extend parallel to the central axis of the brake drum. As braking continues, these ridges create hot spots where the thermal stress exceeds the tensile strength of the material and causes fractures. When the brake drum cools, these fractures widen and become visible as hairline cracks, often referred to as “heat checks.” Heat checks are frequently formed and then worn away as part of the normal braking process. However, in some cases heat check fractures may continue to grow and eventually pass through the wall of the brake drum, causing drum failure.
Gray iron possesses good thermal conductivity and, therefore, resists development of heat checks. In addition, heat checks grow relatively slowly in gray iron and, therefore, are more likely to be worn away before they have an opportunity to progress significantly. In contrast, ductile iron's poor thermal conductivity makes it impractical as a material for brake drums due to its susceptibility to thermal stress and failure caused by heat check cracks, despite the fact that ductile iron has a much higher tensile strength than gray iron.
High tensile strength compacted graphite iron alloys have been developed that have physical characteristics intermediate between gray iron and ductile iron, as disclosed in U.S. Pat. Nos. 6,572,172 and 5,858,127. Compacted graphite has a shape intermediate between flake and nodular graphite, which provides increased tensile strength in comparison to gray iron and improves thermal conductivity relative to ductile iron. Nonetheless, compacted graphite iron brake drums remain much more susceptible to failure caused by thermal stress cracks than gray iron brake drums. Increasing the carbon equivalent in such alloys improves thermal conductivity, but reduces tensile strength. In addition, these compacted graphite irons are harder to produce with consistency, in addition to posing various process constraints.
Other attempts to improve tensile strength while retaining good thermal conductivity include composite products of steel and gray iron, such as brake drums commercially available as SteelLite (ArvinMeritor, Inc.—Troy, Mich.) and CentriFuse Lite (Hayes Lemmerz Int'l, Inc.—Northville, Minn.). Such brake drums have an outer steel shell supporting a gray iron inner liner, that provides a braking surface with good thermal conductivity. Although such composite brake drums provide good resistance to heat checking, they are not as durable as conventional gray iron or compacted graphite iron brake drums, in part due to the problem of separation at the interface between the gray iron liner and steel shell. In addition, the process of forming the gray iron liner within the steel shell requires centrifugal casting equipment and other tooling. Accordingly, composite steel/gray iron brake drums can be more expensive to produce than conventional gray iron brake drums.
Thus, it is desirable to provide a gray iron alloy having increased tensile strength, without sacrificing the mechanical and physical properties of gray iron, such as good thermal conductivity and machineability. In addition, if such an improved gray iron alloy could be found which also provided improved vehicle deceleration when incorporated into brake drums, a particularly significant invention would be had.